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一、R objects

Two basic types of things/objects: data and functions

• Data: things like 6, “six”, $6.000$, and $\left[\begin{array}{ccc}6& 6& 6\\ 6& 6& 6\end{array}\right]$
• Functions: things like log, + (takes two arguments), < (two), sum (multiple), and mean (one)

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二、Types of data

• Booleans
• Integers
• Characters
• Floating point numbers: an integer times a positive integer to the power of an integer, as in $3\times 10^6$ or $1\times 3^{-1}$
  • Double precision: 64 bits (8 bytes); Single precision: 32 bits (4 bytes)
• Missing or ill-defined values: NA, NaN, etc.

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三、Operators

1、Operators

• Unary: take just one argument. E.g., - for arithmetic negation, ! for Boolean negation
• Binary: take two arguments. E.g., +, -, *, and / (though this is only a partial operator). Also, %% (for mod), and ^ (again partial)

• 一元运算符：只接受一个参数。例如，负号（-）用于算术取反，感叹号（!）用于布尔取反。
• 二元运算符：接受两个参数。例如，加号（+）、减号（-）、乘号（*）和除号（/）（尽管除号只是部分运算符）。还有求余运算符（%%），以及幂运算（^）（同样是部分运算符）。

# %/% 执行整除运算，即只保留商的整数部分，舍弃余数。
# %% 是取模运算符，它返回除法后的余数。
# %/% 和 %% 常配合使用来获取整数商和余数。

10 / 3   # 结果是 3.333333
10 %/% 3 # 结果是 3（商的整数部分）
10 %% 3  # 结果是 1（余数）

2、Comparison operators

These are also binary operators; they take two objects, and give back a Boolean.

6 > 5
6 < 5
6 >= 7

3、Logical operators

These basic ones are & (and) and | (or).

(5 > 7) & (6 * 7 == 42)
## [1] FALSE

(5 > 7) | (6 * 7 == 42)
## [1] TRUE

Note: The double forms && and || are different!

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四、Check types of data objects

• The typeof() function returns the data type
• is.x() functions return Boolean values indicating whether the argument is of type x

typeof(6)
## [1] "double"
is.double(7)
## [1] TRUE
## 解释：在 R 语言中，数字的默认类型是 双精度浮点数（double）。

is.na(7)
## [1] FALSE
is.na(7/0)
## [1] FALSE
## 解释：跟数学里不同，7/0 = Inf  

is.na(0/0)
## [1] TRUE

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五、Convertion between data objects

• as.x() (tries to) “cast” its argument to type x, to translate it sensibly into such a value

as.character(5/6)
## [1] "0.833333333333333"

as.numeric(as.character(5/6))
## [1] 0.8333333

6 * as.numeric(as.character(5/6))
## [1] 5

5/6 == as.numeric(as.character(5/6))
## [1] FALSE

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六、R workspace

Where do the variables live?

ls()
## [1] "approx.pi" "circumference" "diameter"

Getting rid of variables:

rm("circumference")
ls()
## [1] "approx.pi" "diameter"

rm(list=ls()) # Be warned! This erases everything
ls()
## character(0)